http2: client conn pool abstraction

Change-Id: Icbf40b26a25c7084efd062a0a66385450ec537aa
Reviewed-on: https://go-review.googlesource.com/16699
Reviewed-by: Blake Mizerany <blake.mizerany@gmail.com>

http2/client_conn_pool.go

@@ -0,0 +1,118 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Transport code's client connection pooling.
+
+package http2
+
+import (
+	"net/http"
+	"sync"
+)
+
+// ClientConnPool manages a pool of HTTP/2 client connections.
+type ClientConnPool interface {
+	GetClientConn(req *http.Request, addr string) (*ClientConn, error)
+	MarkDead(*ClientConn)
+}
+
+type clientConnPool struct {
+	t  *Transport
+	mu sync.Mutex // TODO: switch to RWMutex
+	// TODO: add support for sharing conns based on cert names
+	// (e.g. share conn for googleapis.com and appspot.com)
+	conns map[string][]*ClientConn // key is host:port
+	keys  map[*ClientConn][]string
+}
+
+func (p *clientConnPool) GetClientConn(req *http.Request, addr string) (*ClientConn, error) {
+	return p.getClientConn(req, addr, true)
+}
+
+func (p *clientConnPool) getClientConn(req *http.Request, addr string, dialOnMiss bool) (*ClientConn, error) {
+	p.mu.Lock()
+	for _, cc := range p.conns[addr] {
+		if cc.CanTakeNewRequest() {
+			p.mu.Unlock()
+			return cc, nil
+		}
+	}
+	p.mu.Unlock()
+	if !dialOnMiss {
+		return nil, ErrNoCachedConn
+	}
+
+	// TODO(bradfitz): use a singleflight.Group to only lock once per 'key'.
+	// Probably need to vendor it in as github.com/golang/sync/singleflight
+	// though, since the net package already uses it? Also lines up with
+	// sameer, bcmills, et al wanting to open source some sync stuff.
+	cc, err := p.t.dialClientConn(addr)
+	if err != nil {
+		return nil, err
+	}
+	p.addConn(addr, cc)
+	return cc, nil
+}
+
+func (p *clientConnPool) addConn(key string, cc *ClientConn) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if p.conns == nil {
+		p.conns = make(map[string][]*ClientConn)
+	}
+	if p.keys == nil {
+		p.keys = make(map[*ClientConn][]string)
+	}
+	p.conns[key] = append(p.conns[key], cc)
+	p.keys[cc] = append(p.keys[cc], key)
+}
+
+func (p *clientConnPool) MarkDead(cc *ClientConn) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	for _, key := range p.keys[cc] {
+		vv, ok := p.conns[key]
+		if !ok {
+			continue
+		}
+		newList := filterOutClientConn(vv, cc)
+		if len(newList) > 0 {
+			p.conns[key] = newList
+		} else {
+			delete(p.conns, key)
+		}
+	}
+	delete(p.keys, cc)
+}
+
+func (p *clientConnPool) closeIdleConnections() {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	// TODO: don't close a cc if it was just added to the pool
+	// milliseconds ago and has never been used. There's currently
+	// a small race window with the HTTP/1 Transport's integration
+	// where it can add an idle conn just before using it, and
+	// somebody else can concurrently call CloseIdleConns and
+	// break some caller's RoundTrip.
+	for _, vv := range p.conns {
+		for _, cc := range vv {
+			cc.closeIfIdle()
+		}
+	}
+}
+
+func filterOutClientConn(in []*ClientConn, exclude *ClientConn) []*ClientConn {
+	out := in[:0]
+	for _, v := range in {
+		if v != exclude {
+			out = append(out, v)
+		}
+	}
+	// If we filtered it out, zero out the last item to prevent
+	// the GC from seeing it.
+	if len(in) != len(out) {
+		in[len(in)-1] = nil
+	}
+	return out
+}